Recently, we have demonstrated that as an experimental subject, the rabbit readily develops hydrops following surgical destruction of the endolymphatic duct and sac. The studies proposed here are designed to take advantage of the rabbit model to obtain details of the audiometric changes accompanying surgically induced endolymphatic hydrops in animal ears. These goals will be accomplished using the rabbit conditioned nictitating membrane response that has been well established by behavioral psychologists and modified by us for long-term psychoacoustic testing. Auditory sensitivity functions will be evaluated before and at specified time periods following surgical induction of hydrops. The pattern of audiometric deficit will be related to the precise location and magnitude of hydrops using quantitative morphometry of cochlear scalae of control and hydropic ears obtained from midmodiolar sections of celloidinembedded temporal bones. The ears will also be examined for evidence of ruptures of Reissner's membrane and patterns of hair-cell, spiral-ganglion cell, and nerve-fiber degeneration. Other functional changes to be studied include fluctuations in hearing sensitivity, threshold shifts induced by osmotic agents, and changes in vestibular responsiveness to caloric stimulation. The proposed investigations will provide the baseline data necessary for future studies of basic and clinically relevant aspects of the influence of endolymphatic hydrops on inner-ear function.